The present invention relates to a fluid-driven apparatus, such as a free-piston system, for applications including liquid rocket propulsion; particularly to a control valve assembly for fluid-driven free-piston devices, and more particularly to a valve arrangement using a plurality of valve assemblies for each reciprocating device of an associated pair of such devices so that the devices are alternately actuated by using fluid pressure communication therebetween.
Liquid rocket propulsion requirements have historically fallen into two distinct categories, which correspond to two different aerospace applications. The first kind of requirement is for performance-oriented rocket stages which deliver high levels of thrust continuously with large total impulses, typically used for launch vehicles. High performance is achieved through the use of rotating propellant pumps driven by turbines, which permits high-pressure propellant delivery from relatively lightweight tankage at low pressure. The second kind of requirement is for relatively small propulsion systems, which must reliably provide thrust on demand over periods of many years. The main application of the small systems is orbital maintenance, so performance has been much less important than long-term reliability for this second kind of propulsion system. Hence, satellite liquid propulsion systems have been pressure-fed. Advantages of avoiding pumps are overall simplicity and the lack of moving seals which could wear and permit propellant leakage losses during long periods of in operation. Perhaps more importantly, turbopumps cannot support short thrust pulses, since it requires time at reduced efficiency for turbomachinery to start and stop.
Recently, there has been interest in developing technology for increasing the performance capabilities of small rocket propulsion systems which thrust intermittently. For example, electrically-driven rotating pumps for possible use on satellites have been demonstrated. An approach for high thrust missions has been to increase the performance of pressure-fed operation, by using state-of-the-art materials technology for otherwise mass-intensive pressure vessels and high pressure liquid tankage.
An alternative small propulsion system has been demonstrated that uses low pressure tankage and free-piston pumps which can start and stop rapidly to meet a demand-thrust requirement. These pumps are driven by a gas source. Free-piston devices are positive displacement fluid power machines which undergo reciprocating motion, without mechanical power transfer, such as a connecting rod to a rotating crankshaft. Earlier applications of the free-piston devices were steam-driven air compressors, compressors driven directly by oscillating electromagnetic fields and hydraulic or gas-driven intensifiers. These prior applications generally used a relatively slow-moving differential free-piston to amplify pressure. In contrast, the free-piston pump for small propulsion systems must operate as a high volume flow device, with a high power-to-weight ratio, as well as being a pressure amplifier. U.S. Pat. No. 5,026,259, issued Jun. 25, 1991, in the name of John C. Whitehead et al., exemplifies the use of free-piston devices for propulsion systems used for attitude control or maneuvering.
While gas-driven free-piston pumps can operate at any flow rate, prior known propulsion systems using free-piston pumps have been unable to provide continuous flow of either monopropellant or bipropellant for rocket systems without complicated electrical control systems. Thus, there has existed in the small rocket propulsion systems a need for a valve control system which provides for rapid response, as well as continuous liquid delivery for steady thrust.
The present invention fills this prior recognized need by providing an improved valve control assembly which alternately actuates a pair of fluid-driven pumps, using fluid pressure communication between them. This invention is an improvement over the three-way valve arrangements of the above-referenced application Ser. No. 07/901,290, now U.S. Pat. No. 5,222,873, in that the intake-exhaust valve and the signal valve are separated, which greatly simplifies the valve assembly and provides fewer critical leakage paths, among other advantages. Each control valve of one of a pair of pumps is switched by a pressure signal depending on the state of its counterpart's pump piston. The communication logic is arranged to provide overlap of the forward strokes of the pistons, so that at least one of the pair of pumps will always be pressurized. Thus, uninterrupted pumping of liquid is made possible from a pair of free piston pumps. In addition, the speed and frequency of piston stroking is entirely dependent on the mechanical power load applied. In the case of a pair of pumps, this enables liquid delivery at a substantially constant pressure over the full range of flow rates, from zero to maximum flow.
The present invention like that described and claimed in the above-referenced has application for both monopropellant liquid propulsion systems and for bipropellant liquid propulsion systems, each utilizing reciprocating pumps. For the bipropellant system, a total of four pumps enables continuous flow of both fuel and oxidizer liquids, while allowing a different flow rate of each of the different liquids utilized.